Using a LDO regulator as a CC source...
Inducktion, Fri Dec 30 2011, 09:19PM
I have the adjustable version of this.... I'm trying to figure out how to make it a CC source, but it doesn't seem to be working. With 5 ohms of resistance from Vout to Vadjust, I get .5 amps of current flow with 4 volts in... With 9 volts, it jumps up to 1 amp, which doesn't make sense. It doesn't seem to be regulating, at all, with all of the current flowing straight in from Vin to Vout through the resistors.
Is it just not possible to make CC sources with LDO regulators?
Re: Using a LDO regulator as a CC source...
Pinky's Brain, Fri Dec 30 2011, 09:39PM
Silly question ... the 5 ohm is in series with the load right?
Pinky's Brain, Fri Dec 30 2011, 09:39PM
Silly question ... the 5 ohm is in series with the load right?
Re: Using a LDO regulator as a CC source...
Inducktion, Fri Dec 30 2011, 10:12PM
I put the output wire from Vadjust to the load. So, yes, the resistors are in series with the load, since it goes Vout -> 5 ohms -> Vadjust -> load
Inducktion, Fri Dec 30 2011, 10:12PM
I put the output wire from Vadjust to the load. So, yes, the resistors are in series with the load, since it goes Vout -> 5 ohms -> Vadjust -> load
Re: Using a LDO regulator as a CC source...
klugesmith, Fri Dec 30 2011, 11:03PM
Uh, how are you thinking the circuit is supposed to work?
The regulator IC does its best to maintain 1.25 V between its Vadj and Gnd pins.
Assuming you want 0.25 A, the 5 ohm current sense resistor belongs between those pins.
The traditional place to put the regulated-current load (a jumper wire would qualify) is between the Vout and Vadj pins.
There are two other places where you could put a load instead of a wire:
1) between the Gnd pin and the negative terminal of the power source.
2) between the positive terminal of the power source and the Vin pin.
They each have the advantage of one node in common with the power source.
And the disadvantage of carrying the regulator's GND-pin current (a few mA),
in addition to the regulated current in the sense resistor.
If you wanted to use a fixed-voltage regulator with no Vadj pin,
you would have to put the load in one (or both) of those locations.
Note that there are LDO regulators whose reference voltage is 0.5 V instead of 1.25 V.
klugesmith, Fri Dec 30 2011, 11:03PM
Uh, how are you thinking the circuit is supposed to work?
The regulator IC does its best to maintain 1.25 V between its Vadj and Gnd pins.
Assuming you want 0.25 A, the 5 ohm current sense resistor belongs between those pins.
The traditional place to put the regulated-current load (a jumper wire would qualify) is between the Vout and Vadj pins.
There are two other places where you could put a load instead of a wire:
1) between the Gnd pin and the negative terminal of the power source.
2) between the positive terminal of the power source and the Vin pin.
They each have the advantage of one node in common with the power source.
And the disadvantage of carrying the regulator's GND-pin current (a few mA),
in addition to the regulated current in the sense resistor.
If you wanted to use a fixed-voltage regulator with no Vadj pin,
you would have to put the load in one (or both) of those locations.
Note that there are LDO regulators whose reference voltage is 0.5 V instead of 1.25 V.
Re: Using a LDO regulator as a CC source...
Inducktion, Fri Dec 30 2011, 11:57PM
Thanks Kludge. I finally got it working; The laser diode is happy with 250 mA off current, and is lasing perfectly. No difference in intensity between 9 volts and 4.5 volts.
Inducktion, Fri Dec 30 2011, 11:57PM
Thanks Kludge. I finally got it working; The laser diode is happy with 250 mA off current, and is lasing perfectly. No difference in intensity between 9 volts and 4.5 volts.
Re: Using a LDO regulator as a CC source...
Inducktion, Sat Dec 31 2011, 04:56AM
I do apologize for the double post;
However,
Does it drop 1.75 volts as rhd suggests? The drop out voltage of the regulator itself is .5 volts which is really low, but the Vref of the regulator is 1.25 volts. The diode has a voltage drop as well though.
Inducktion, Sat Dec 31 2011, 04:56AM
I do apologize for the double post;
However,
Does it drop 1.75 volts as rhd suggests? The drop out voltage of the regulator itself is .5 volts which is really low, but the Vref of the regulator is 1.25 volts. The diode has a voltage drop as well though.
Print this page